1. Field of the Invention
The subject matter disclosed generally relates to high speed analog to digital converters used for detectors such as mass spectrometers and light ranging detectors.
2. Background Information
There is a critical need to develop electronic detection systems capable of recording high-speed transient events. High speed transient detectors typically utilize digitization techniques to process the data. One of the major challenges in high-speed digitization is maximizing the dynamic range of detection, which is generally defined as the measurable range spanning the limit of weak to strong signal. Assuming low analog noise, the minimum signal level detectable by a transient digitizer is generally on the order of one ion or photon per time bin per trigger event. Transient digitizers are considered to operate in what is called the strong signal limit, usually defined as a detected flux of ions or photons corresponding to multiple events per time resolution element (or time bin). Weaker signals are generally masked by the analog noise arising from the detector, amplifier, and digitizer of the detector.
The weak signal regime is defined as the detection of less than one ion or photon count per bin per trigger event. In this regime, it is necessary to use pulse detection for individual ion or photon counts, where each pulse is timed and deposited as a bit in a time bin measured by a multichannel scaler (MCS). To achieve high-speed operation, pulses are detected as binary events, returning either a 0 or 1. This leads to the potential for counting error when multiple pulses are detected as one pulse. To keep the probability of multiple pulses per time bin per trigger event to an acceptable level, the maximum counting probability per time bin is typically limited to about 0.1.
High dynamic range detectors often use both transient digitizer and MCS/averager systems. However, this method has several drawbacks: (1) such a system is expensive and complicated, (2) it requires routines to recognize whether a signal is in the strong or weak limit, (3) it must seam the data together from the two detection systems, and (4) the signal regime between 0.1 and 1 count per trigger event per bin is inadequately measured by either method.
Another detection method employs a threshold condition for a digitizer (threshold digitizer). However, the routine does not keep track of the baseline value and hence intensity errors can occur as shown in FIG. 1. For example, a DC drift in the signal may raise portions of the noise above the threshold and produce errant signals. Another method is also based on a transient digitizer and achieves noise elimination by offsetting the signal so that the analog noise appears below the minimum scale (offsetting digitizer). The minimum scale then defines the threshold for signal and the minimum value. Again, the true baseline is not recorded and intensity errors can result as illustrated in FIG. 1.
U.S. Pat. No. 5,568,144 issued to Chiao et al. discloses a method to extend the dynamic range for recording waveforms. The method uses a threshold to distinguish a weak signal from a strong signal. The waveform is directly measured for the weak signal and the strong signal is measured using a differential process. This method is more suitable for repetitive waveform analysis. It is not applicable to the case where the weak signals are individual ion or photon counts and where each transient response is different.
U.S. Pat. No. 5,138,552 issued to Weedon et al. discloses a data acquisition system that uses non-linear digitization intervals to expand dynamic range. Again, this method is for waveforms and not single counts. U.S. Pat. No. 5,422,643 issued to Chu et al. discloses a high dynamic range digitizer that is based on a plurality of channels. The channels receive a signal that is passed through a scaling bank to partition the high dynamic range into a number of vertical intensities. Another disclosure based on a multichannel approach is in U.S. Pat. No. 5,068,658 issued to Ohlsson et al. Other multichannel approaches including variable gain have also been disclosed in the patent literature. The latter methods all are based on multiple channels of detection. Multi-channel detectors are relatively expensive to produce.
Other methods have been developed for extending the dynamic range of a detector. U.S. Pat. No. 6,028,543 issued to Gedcke et al. discloses a method based on dithering successive ADC traces by a varying value that is less than the least significant bit. For example, 4 bits of additional resolution may be obtained by dithering the baseline in intervals of {fraction (1/16)} of the least significant bit. However, this method requires at least 16 scans to achieve the resolution enhancement, and is most effective for repetitive signals.
Dynamic range may also be extended by increasing the number of detector segments that can detect a signal. This method enables multiple counts to be detected simultaneously. A disadvantage to this method is that each segment requires its own data system, which makes the overall system complex and expensive to produce. For photon detection, a multicathode photomultiplier tube (PMT) detector allows multiple pulses to be detected at once, on the premise that they are likely to strike different regions of the detector face and hence lead to independent pulses on separate anodes. In theory, the PMT can be divided into more segments to further increase dynamic range.
A detector system that includes a detector, an analog to digital converter and a processor. The detector provides an analog signal. The analog signal is processed to determine a baseline value and threshold value, wherein portions of the signal below or at the threshold are assigned the baseline value.